1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor structure for two-dimensional magnetic recording (TDMR).
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu) or silver (Ag). One ferromagnetic layer adjacent to the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference or pinned layer. The other ferromagnetic layer adjacent to the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the pinned-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
A proposed technology that uses multiple CPP-MR sensors is two-dimensional magnetic recording (TDMR). In TDMR, multiple sensors that are located on a single read head access the same or adjacent data tracks to obtain signals that are processed jointly. This allows the data tracks to be placed closer together, resulting in an increase in areal data bit density. In addition to increasing areal density, TDMR may provide an increased readback data rate if data from multiple data tracks are read concurrently. A structure with multiple stacked read sensors for TDMR is described in US 2013/0286502 A1.
Each of the individual CPP-MR sensors in a TDMR read head is required to be located between two shields of magnetically permeable material that shield the read head from recorded data bits that are neighboring the data bit being read. During readback, the shields ensure that each sensor reads only the information from the targeted disk region.
In a TDMR sensor structure, such as a structure with two stacked read sensors, it is desirable to minimize the free layer to free layer spacing between the two read sensors. This requires that the center shield between the two sensors be made as thin as possible. An additional problem arises if the free layer of the lower read sensor has its magnetization magneto-statically biased by side shields of soft magnetic material. An antiferromagnetic layer is needed to pin the magnetization of the center shield in a direction substantially parallel to the ABS. Since the center shield is magnetically coupled to the side-shields their magnetization is pinned substantially parallel to the ABS as well, assuring the stabilization of the free layer. Because the reference or pinned layers of the two read sensors also have their magnetizations pinned by antiferromagnetic layers, but in a direction substantially perpendicular to the ABS, i.e., orthogonal to the magnetization of the center shield, four sequential annealing steps are required. A first annealing step at high temperature pins the magnetization of the lower sensor's pinned layer in one direction, the second annealing step at lower temperature pins the magnetization of the center shield pinned layer in an orthogonal direction, the third annealing step at high temperature pins the magnetization of the upper sensor's pinned layer in the same direction as the magnetization of the lower sensor's pinned layer and will upset the previously set magnetization of the center shield pinned layer. Thus a fourth annealing step at lower temperature is required to reset the magnetization of the center shield. This fourth annealing step however will not be able to fully reset the pinned layer magnetization of the center shield. For this the temperature would have to be increased, which would result in upsetting the magnetization of the lower and upper sensors' pinned layers.
What is needed is a stacked CPP-MR sensor structure for TDMR that has reduced free layer to free layer spacing and that does not require an annealing sequence that upsets previously set magnetizations.